1. Field of the Invention
This invention relates to a method for producing a polyolefin foam such as a polyethylene foam or polypropylene foam and a method for improving its thermal resistance and surface wettability.
2. Description of the Prior Art
Polyolefin foam sheets such as polyethylene foam sheets and polypropylene foam sheets have conventionally been utilized for various uses such as vehicle interior materials, housetop heat insulating materials, crating materials and various buoyancy materials.
The polyolefin foam sheet is usually produced by heating a foamable base sheet containing a polyolefin resin and an organic foaming agent which is solid at room temperature and decomposes and generates gas during heating to foam the base sheet. In this case, the polyolefin resin of the base sheet is crosslinked before heating and foaming to give a proper melt viscosity thereto and the gas generated due to decomposition of the foaming agent by heating produces many foams against the viscosity of the polyolefin resin of the base sheet, which results in the foaming of the base sheet thereby producing a polyolefin foam sheet. The base sheet is converted into a polyolefin foam sheet having an expansion ratio in accordance with the amount of the foaming agent contained in the base sheet by properly controlling the heating temperature and the heating time. Use of a long size base sheet results in a long size foam sheet after heating.
However, in the conventional production of polyolefin foam sheets, the permissible ranges of heating temperature and heating time for producing foams having good appearance and physical properties are very narrow and good foam sheets can be obtained only within narrow temperature and time ranges. Therefore, in producing a foam sheet by heating a base sheet containing a polyolefin resin and a foaming agent according to the conventional method, the thickness of the base sheet should be made accurate and the heating temperature and the heating time should be precisely controlled in order to obtain a polyolefin foam sheet having a desired thickness and desired physical properties. Variations of heating temperature and time may result in foam melting, non-foaming, inferior appearance of the resulting foam sheet such as its total width variation, and reduced physical properties of the foam sheet. Therefore, the conventional method for producing polyolefin foam sheets has many problems on foaming process.
Conventional polyolefin foams are not sufficiently satisfactory in terms of their thermal resistance. For instance, the conventional polyolefin foam may be melted to deform easily at high temperature.
In addition, conventional polyolefin foams have inferior surface wettability. When such a conventional polyolefin foam is laminated with an outer layer such as a vinyl chloride sheet in a later process, sufficient adhesive strength can not be obtained if the surface of the polyolefin foam has not been activated by corona discharge or a similar treatment.
Since the surface improvement by corona discharge is a temporary improving treatment, the surface wettability decreases with the passing of time and sufficient adhesive strength may no longer be obtained two weeks after the surface treatment. It is difficult to give stable adhesion over a long period by corona discharge.